feat(http-tracker): evaluate and implement native HTTP/3 support #1765

Description

Overview

This is a follow-up to #1736. Once upstream Rust HTTP dependencies (Hyper, Axum) provide stable HTTP/3 support, evaluate and test native HTTP/3 support in the tracker HTTP server, document results, performance impact, and any required code changes or configuration additions.

Current blocker: The Rust HTTP ecosystem (Hyper, Axum) is still stabilizing HTTP/3 support (see hyperium/hyper#3925).

Why this issue

1. Native HTTP/3 in the tracker would simplify deployments (no reverse proxy required for HTTP/3).
2. The project should avoid losing track of native HTTP/3 adoption once upstream dependencies mature.

Scope

1. Test functional correctness of native HTTP/3 on tracker announce/scrape endpoints and REST API.
2. Benchmark performance and resource usage (CPU, memory) of direct HTTP/3 vs. proxy-terminated HTTP/3.
3. Document migration path and backward compatibility requirements.
4. Update tracker HTTP server code and deployment docs if native HTTP/3 support is viable.

Test/benchmark checklist

• Confirm upstream HTTP/3 support is stable (Hyper, Axum)
• Verify announce endpoint works over HTTP/3
• Verify scrape endpoint works over HTTP/3
• Verify REST API routes work over HTTP/3
• Benchmark CPU and memory usage vs. proxy-terminated HTTP/3
• Benchmark latency vs. proxy-terminated HTTP/3
• Document migration path and backward compatibility requirements
• Update deployment docs with native HTTP/3 configuration examples

Acceptance criteria

• Upstream HTTP/3 support status is confirmed stable (documented in issue comments).
• Functional tests confirm HTTP/3 works correctly for all tracker endpoints.
• Performance benchmarks are documented.
• A clear migration path is documented.
• If native HTTP/3 is not viable, rationale is documented in a comment.

References

• Parent issue: docs(http): document HTTP/3 support via reverse proxy and track native HTTP/3 follow-up #1736
• Upstream tracking: WIP: Add HTTP/3 support hyperium/hyper#3925
• Demo HTTP/3 issue: docs(issue): re-enable caddy http3 and document issue-29 rationale torrust-tracker-demo#31